The following assumes you are able to build OpenCPN from source and have the needed tools. You can find the detailed instructions at Compiling Linux and Compiling Windows
If you followed the building instructions above, you already have the local repository clone and all the needed tools on your computer. Open a terminal (run cmd.exe) and change to the directory where you have cloned the source.
Personal advice: to save a lot of problems later, don't be afraid to use a lot of small per-task branches. Could sound weird especially to the guys used to the traditional version control systems, but this is how it's meant to be with git and modern distributed version control systems. And believe me that merging branches is much easier than cherry-picking commits belonging to a single task.
Following are a few git commands you will for sure need sooner or later:
To update to the latest upstream code
git pull origin
To see all the branches in your local repository
git branch
To create a new branch
git branch [NAME]
To change to another branch (don't forget to commit or reset your changes before changing to another branch, otherwise git will not be happy throwing away your work and overwriting it with other branch)
git checkout [NAME]
To delete a branch
git branch -D [NAME]
To add a file to the current branch
git add [filename.ext]
To delete a file from the current branch
git rm [filename.ext]
To commit your local changes to the current branch
git commit -a -m "My commit message text"
To throw away the latest code changes you've done and get back to the last commited state
git reset --hard
Be aware that this is not a Git tutorial at all. To know how to use Git, consult the Git book at http://book.git-scm.com/ or any other tutorials on the net.
Please read the page on Code Formatting before writing new code for OpenCPN.
The best way to ensure your code gets properly submitted to OpenCPN is to create a clone of the OpenCPN GitHub master repository, and push your commits to your cloned repository. When everything is available on Github you can either send a mail the developers or create an issue in http://opencpn.willkamp.com describing your work and referenceing the github commit URL.
The official development master is here: https://github.com/OpenCPN/OpenCPN
Expecting your branch is checked out and you are in your working directory. Simply issue:
git diff master> mystuff.patch
In the newly created file called mystuff.patch you may review the code you are going to submit with any text editor.
Create a Flyspray account at http://opencpn.willkamp.com
Attach the patch with comment to the corresponding issue in the bugtracker - for 4.5.x http://opencpn.willkamp.com/index.php?project=76&do=index&switch=1
Attach the patch with comment to the corresponding issue in the Feature requests tracker at http://opencpn.willkamp.com/index.php?do=newtask&project=6